It has been proposed to use Fiber Bragg Grating (FBG) sensor technology employing optical fibres which may be embedded in or attached to structures, to provide real time strain, temperature and structural integrity information of the structures. The major cost of such technology lies in the interrogation and demultiplexing units for such sensors rather than in the sensors themselves. The majority of the interrogation techniques so far developed have been based on optical filtering methods employing tuneable filters or interferometers which techniques are relatively expensive to implement and relatively cumbersome. It has also been proposed to use matched Fibre Bragg Gratings as a receiving device to track the sensing structures. However this latter method requires the use of a number of expensive piezoelectric actuators to drive the receiving gratings which once again is a relatively expensive technique.